The invention relates to a dampening unit for a printing press having a first roller and a second roller, which together form a slip gap.
One such dampening unit is described in Published, European Patent Application EP 0 893 251 A2; it includes an immersion roller, a slip roller, and an intermediate roller. The slip roller rotates with a surface velocity that is different from a surface velocity of the intermediate roller. As a consequence, a roller gap formed by the slip roller together with the intermediate roller is a so-called slip gap. The afore-mentioned rollers carry only humectant, and not an emulsion of printing ink and humectant.
U.S. Pat. No. 4,949,639 also describes another dampening unit, which includes a plurality of rollers each of which contains rubber and rotates at the surface velocity of a plate cylinder. All the rollers of the dampening unit carry humectant and printing ink during printing. The dampening unit is thus a so-called direct-film or emulsion-film dampening unit.
In the brochure xe2x80x9cPrinting without Isopropanol (IPA)xe2x80x9d, in the brochure series xe2x80x9cGTO 52-Tipsxe2x80x9d, issued by Heidelberger Druckmaschinen AG, a further direct-film dampening unit is mentioned, for whose operation and care tips are given in the brochure.
Other prior art is also described in U.S. Pat. No. 5,540,145, Published, European Patent Application EP 0 462 490 A1, Published, Non-Prosecuted German Patent Applications DE 36 37 460 A1 and DE 37 22 519 A1, and German Patent DE 43 12 523 C2.
It is accordingly an object of the invention to provide a dampening unit for a printing press which overcomes the above-mentioned disadvantages of the prior art devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a dampening unit for a printing press, including a first roller having a center axis; and a second roller jointly forming a slip gap with the first roller, the first roller being swivably mounted about a pivot axis for positioning the first roller in relationship to the second roller, the pivot axis extends through the center axis of the first roller and is substantially parallel to a tangential line running through the slip gap.
One of the advantages of the dampening unit of the invention is that a shear force occurring in the slip gap cannot engender any torque that pulls the first roller from the second roller or forces it toward the second roller. As a consequence, the roller pressure in the slip gap and the film of humectant, or preferably an emulsion of printing ink and humectant, fed through it remain constant.
In addition, the dampening unit of the invention is highly suitable as an emulsion film dampening unit for processing an alcohol-free humectant, which in turn is ecologically advantageous. Experience shows that if an emulsion of printing ink and humectant is sheared in an emulsion film dampening unit, a greater shear force occurs than when an ink-free humectant film is sheared in a so-called alcohol dampening unit. The high shear forces, in the emulsion dampening units known from the prior art, dictate structural and functional limitations. In contrast to this, such limitations no longer exist in the dampening unit of the invention.
For instance, it is easily possible in the dampening unit of the invention to assign the first roller a force storing device, such as a spring, for storing a positioning force that pivots the first roller against the second roller about the pivot axis. The use of such a force storing device that elastically holds the first roller against the second roller and that acts resiliently upon a slight depositioning motion of the first roller away from the second roller, is extremely advantageous with a view to precise metering of an emulsion of printing ink and humectant.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a dampening unit for a printing press, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.